U.S. Pat. No. 4,627,474 WO84/01394 already discloses a very sophisticated technique for controlling a yarn storing, feeding and measuring device. More particularly, this patent discloses a method for controlling a yarn storing, feeding and measuring device for jet looms, having a stationary storage drum onto which a yarn store can be wound by a winding-on device and from which the yarn can be withdrawn, spiralling around the withdrawal end of the storage drum. The device further comprises yarn sensing means arranged such that the yarn is passing a detection area during its withdrawal from the drum which produce pulse signals, each pulse signal indicating a passing of the yarn through the detection area of the sensing means. In addition, it includes a plurality of electromagnetic yarn stopping devices which are arranged at regular angular intervals around the storage drum. Each electromagnetic stopping device includes an electromagnetic coil which can be energised by means of an actuation current generated by a control unit as well as a stopping element which can be moved into the withdrawal path of the yarn when energising the coil for stopping the withdrawal of yarn from the drum. The control unit of the prior art yarn storing, feeding and measuring device includes a memory for storing the positional number of the stopping device actuated at the end of a preceding yean withdrawal cycle which releases the yarn at the beginning of a present yarn withdrawal cycle. On the basis of said information, regarding the positional number, the control unit determines the positional number of the stopping device to be actuated next on the basis of information, regarding the desired yarn length corresponding to the so-called shot length for one weft yarn insertion shot. In other words, the control unit determined how many turns are to be withdrawn from the drum for achieving a desired yarn length, determines the number of complete turns and determines the fraction of the last turn which is necessary for obtaining the desired yarn shot length. Hence, the positional offset of the next yarn stopping device, with respect to the preceding yarn stopping device, can be derived from said fraction of one turn for obtaining information, regarding the positional number of the stopping device to be actuated next. After releasing the previously actuated stopping device, the control unit measures the time from the moment of releasing, derives the actual withdrawn yarn lengths therefrom and periodically adapts the calculated withdrawn yarn length to the actually withdrawn yarn length each time the yarn passes a detection area of the yarn sensor. Assuming that the desired yarn length corresponds to eight and one-half turns of yarn, the calculated yarn length, corresponding to the calculated time since the releasing of the stopping device actuated at the end of a preceding withdrawal cycle, is brought into conformity with the actual length at each passing of the yarn through the detection area of the sensor, e.g. eight times in the present example. After receipt of the eighth pulse from the yarn sensor, the prior art control unit actuates the yarn stopping device, having the positional number which has been derived from the positional number of the yarn stopping device actuated at the end of the previous cycle. However, the prior art system is not adapted for extreme high speeds of the weft yarn during the insertion shot or for controlling a storing, feeding and measuring device, having a storage drum with a very small diameter and, thus, having a very high rotational withdrawal speed at a pre-determined weft yarn insertion velocity.
In view of this state of the art, the present invention is based on the technical task of how to further develop a method of the abovementioned kind so that an exact lengthening of the weft yarn is obtained even when driving the device at extreme velocities.